Communication among a plurality of devices is, for example, conventionally carried out by serial communication. Such devices include an open-drain type output circuit (see Japanese Patent Publication No. 2009-531934).
As illustrated in FIG. 1, a plurality of devices 11, 12, and 13 is mutually coupled by a transmission path 14 which transmits and receives data. The device 13 includes an output circuit 15 which outputs data. Although not illustrated, other devices 11 and 12 also include a similar output circuit.
The output circuit 15 is an open drain type driver circuit. A resistor R1 pulls up a potential level of the transmission path 14. The device 13 activates an N-channel MOS transistor T1, which is coupled to an external terminal P0, to pull down the potential level of the transmission path 14, which is coupled to the external terminal P0. Thus, the voltage Vc of the signal propagated through the transmission path 14 is changed, as illustrated in FIG. 2A.
In this manner, the output circuit 15 activates the N-channel MOS transistor T1 to change the potential of the transmission path 14 from H level to L level. A waveform shaping capacitor C1 may be coupled between a gate and a drain of the transistor T1 to adjust the slope of the falling edge of the potential level in the transmission path 14.
In the above system, when the communication is performed between the two devices 11 and 12, the power supply of the device 13 that does not perform the communication may be turned off. When the power supply of the device 13 is turned off, the supply of high potential voltage VDE to an inverter circuit 16, for example, is stopped. In this case, a gate terminal of the transistor T1 is in a floating state. Furthermore, when the high potential voltage VDE is fixed to a ground potential (0V), the gate terminal of the transistor T1 is held at L level. Thus, when the power supply of the device 13 is turned OFF or the power supply voltage is fixed at the ground potential, the gate terminal of the transistor T1 of the output circuit 15 is AC coupled with the transmission path 14 through the waveform shaping capacitor C1. Therefore, when the potential of the transmission path 14 changes from L level to H level, the gate voltage of the transistor T1 also rises. The transistor T1 is then weakly activated. As a result, as illustrated in FIG. 2B, the waveform of the voltage Vc in the transmission path 14 changes from a waveform indicated by the dotted line to a waveform indicated by the solid line. That is, the transistor T1 of the device 13 illustrated in FIG. 1 draws in minimal current from the transmission path 14, so that the waveform shape of the signal communicated through the transmission path 14 between the device 11 and the device 12 changes undesirably.